As office automation progresses for deskwork, data transfer from a host computer to a printing apparatus and printing documents, drawings, and the like increases more and more. In recent years, it becomes popular to create documents and drawings in color. Printing apparatuses capable of high-speed printing and printing apparatuses capable of color printing are being introduced more and more.
A color printing apparatus is not used to print only color documents though its principal object is to print color documents, and is often used to print monochrome documents. Such a color printing apparatus is utilized to print a document containing color and monochrome pages or to perform printing based on instructions from a plurality of users in a network. The trend of coexistence of color and monochrome documents is growing.
A conventional full-color page printer has both monochrome and full-color print processes. The page printer generally prints a monochrome page by the monochrome print process and a full-color page by the full-color print process for the purpose. of high-speed output of monochrome pages, power saving, and reduction in the wear of a full-color printing mechanism by adopting a print process for monochrome printing and printing a monochrome page in the monochrome print process.
In general, a color image forming apparatus such as a printing apparatus, color printer, or color copying machine has an engine for printing (image transfer) by assembling (unitizing) developing units for a total of four toners: K (black) toner used to print characters and the like, and Y (Yellow) toner, M (Magenta: red dye) toner, and C (Cyan: greenish blue) toner, which are three primary colors in subtractive color mixture.
In image transfer, the respective color units are operated to multi-transfer the respective color toners onto a print sheet on the basis of pixel information mapped in frame memories for the respective colors.
In this color printer, a document to be printed (image transfer) is not limited to a color document. In practice, the color printer is used to print both color and monochrome documents.
Generally, a monochrome document can be printed using only a K toner process unit (to be referred to as a monochrome process unit hereinafter: the remaining process units will be referred to as a color process unit hereinafter).
In order to prevent degradation of the process unit by toner consumption, friction, and the like, some color printers have a function of, when a print document is monochrome, separating the color process unit so as not to operate it, and operating only the monochrome process unit, thus preventing consumption of the color process unit.
Note that “operating the process unit” includes a state in which the process unit runs idle without any substantial printing, like the color process unit during monochrome printing in a printer not equipped with the above function.
If the above-mentioned printer can suppress the wear and power consumption of the full-color printing mechanism to reduce the printing cost of a monochrome page in a full-color printer close to the printing cost in a monochrome printer in printing a monochrome page, the user neither installs both monochrome and full-color printers nor selects the type of printer in accordance with each document to be printed.
The prior art of using the monochrome and full-color print processes and switching the print process, as needed is therefore indispensable for spreading full-color page printers.
An example of the color printer which switches the print process is a tandem color printer which adopts one of typical processes of a color image forming apparatus. The tandem color printer comprises drums for respective color components, and realizes high-speed print processing by arranging these drums parallel to each other on the convey path and performing transfer of all colors by one process.
More specifically, the tandem color printer has a mechanism which comprises four photosensitive drums (photosensitive drums corresponding to four colors: cyan, magenta, yellow, and black) in order to increase the color printing speed, and transfers an image onto a transfer medium by four developing units.
Some tandem printing apparatuses have a mechanism which operates yellow, cyan, and magenta units not used for printing (e.g., idly operates photosensitive drums). Idle operation of photosensitive drums excessively shortens the service life of units such as yellow, cyan, and magenta photosensitive drums.
In order to reduce toner consumption and the wear of each unit, a color printing apparatus of this type switches the operation between color print processing and monochrome print processing. In printing a monochrome document, the color printing apparatus controls to move yellow, cyan, and magenta developing units except a black developing unit apart from the electrostatic convey belt so as not to print in colors except black.
That is, there has been proposed a color printing apparatus which switches between color print operation and monochrome print operation of retracting the yellow, cyan, and magenta developing units in accordance with supplied data contents so as not to shorten the service life of the yellow, cyan, and magenta units. If the supplied data is only monochrome data, printing is done by automatically switching the operation to monochrome print operation.
As described above, the conventional full-color page printer realizes high-speed output, power saving, and protection of the full-color printing mechanism in printing a monochrome page by switching between the monochrome and full-color page print processes. In general, print process switching operation requires a predetermined processing time, consuming power. In addition, print process switching operation loads the printing mechanism, and frequent switching of the print process shortens the service life of the printing mechanism.
For example, in printing a document in which color and monochrome pages appear almost alternately, or in a use environment where a plurality of users frequently print a document of 1 or 2 pages, monochrome and full-color pages are printed at random, and the print process will be frequently switched. In this case, even if the monochrome print process is higher in speed and lower in power consumption than the full-color print process, the processing time and power consumption in monochrome printing increase owing to the processing time of print process switching operation, power consumption, and print process switching operation. Printing all pages in the full-color print process may become more preferable in terms of the printing speed, power consumption, and load on the printing mechanism.
Shift from monochrome print operation to color print operation or opposite shift requires an operation such as retraction/return of the developing unit due to the difference between monochrome and color print operations. If print operation shifts, data cannot be kept printed, requiring a switching time.
For example, the above-described tandem color printer takes a switching time between color and monochrome print operations. To prevent this, there has been proposed a method of printing monochrome data in color to increase the throughput without switching print operation under a predetermined condition when printing a document containing both color and monochrome pages.
For example, Japanese Patent Laid-Open Nos. 11-34438 and 2000-29266 disclose a method of printing a monochrome page as a target page in the full-color print process under a given condition when an immediately preceding page is printed in the full-color print process.
In Japanese Patent Laid-Open No. 11-34438, print operation is switched in terms of the throughput. That is, there is proposed a printing apparatus in which, when a monochrome image is received as an image to be printed next during the color print mode, the printing time of printing the monochrome image in the color print mode and the printing time of switching to the monochrome print mode and then printing the image are compared, and if the printing time in the color print mode is shorter, the image is printed in the color print mode. According to this method, an engine which takes a very long time for switching from the color print mode to the monochrome print mode and always prints data at a high speed in the color print mode without switching it keeps printing monochrome data in the color print mode once monochrome data is printed in the color print mode. This greatly shortens the service life of a unit such as a photosensitive drum in the tandem printing apparatus or the like.
To the contrary, in the method proposed in Japanese Patent Laid-Open No. 2000-29266, the print mode is switched in terms of toner waste and the service life of each component. This method adopts an approach of calculating a disadvantage generated when monochrome data is printed in the color print mode and a disadvantage generated when the mode is switched to the monochrome print mode, and comparing these disadvantages to decide whether to switch the mode. However, it is difficult to accurately calculate the disadvantage, and when a print mode selected on the basis of the disadvantage is determined to be switched, the throughput may decrease.
These methods are based on switching to the monochrome print process when it is confirmed by referring to the attributes of pages subsequent to a target page that monochrome pages succeed to a given extent, i.e., when switching to the monochrome print process is determined to be advantageous. The attributes of a predetermined number of succeeding pages must be referred to.
Particularly when these methods are applied to a printing mechanism in which the print process switching time is relatively long with respect to the difference in the printing time of one page between the monochrome and full-color print processes, the next target page cannot be printed until the page attributes of many pages are decided. For example, attention is paid to the printing time, and these methods are applied to a printing mechanism in which the monochrome printing speed is 2 sec/page, the full-color printing speed is 3 sec/page, and the time taken to switch the print process is about 10 sec. In this case, when the print process is switched from the full-color print process to the monochrome print process and returns to the full-color print process again, printing upon switching to the monochrome print process does not provide any merit from the viewpoint of the printing time unless the number of successive pages to be printed in the monochrome print process is at least 20 or more.
This means that a target page may not be printed unless 20 or more succeeding pages are accumulated. To accumulate succeeding pages, the printing mechanism may be kept idle.
These references define operation when a page immediately preceding a target page is a full-color page. If no immediately preceding page exists, i.e., print data starting from a monochrome page is to be processed while no print processing is done, the print process may not be optimum depending on the condition.
Switching of the print process based on whether a document is color or monochrome is achieved by separating each process unit from a sheet by mechanical operation. The print mode cannot be switched until the sheet is completely discharged.
One switching operation takes a very long time of about 10 to 30 sec. For this reason, every printing of color and monochrome pages by one job requires a switching time, which impairs high-speed operation which is a merit of the tandem printing apparatus.
This is not limited to a printing apparatus having the tandem printing engine. Also in the use of a single-head apparatus or intermediate transfer member, the print mode is switched by separating each process unit from a sheet by mechanical operation. A long time is similarly taken, and the printing speed decreases in the coexistence of monochrome and color pages.